1. Field of the Invention
The present invention relates to a passive micromixer, which can uniformly mix at least two fluids within a very short distance.
2. Description of the Related Art
Before, mixing was usually applied to the fields of mechanics and chemistry, such as chemical synthesis and combustion engineering. Because the advance in microelectromechanics brings rapid developments of microfluidics, a revolutionary development of biomedical chemistry is further inspired. Dismissing the original complicated biomedical analysis processes, procedures of standardized analysis are integrated onto a lab-on-a-chip or the micro total analysis system. A system integrating with microelectromechanics, biomedical technology, analytical chemistry, and optoelectronics is able to perform a series of test procedures of mixing, separation, and transportation, and has the advantages of small volume, low cost, parallel-processing capability, rapid response and disposability. According to the abovementioned, a micromixer is thus developed for mixing in microscale. And now, improving the mixing performance of micromixers becomes a focus topic in the fields concerned.
The size of a lab-on-a-chip or a micro total analysis system is generally about several centimeters and the width of the microchannel thereof ranges from tens to hundreds of microns; therefore, the Reynolds number of the system is greatly decreased. Reynolds number is defined to be:Re=ρDU/μwherein ρ is the density of the fluid; D is the width of the microchannel; U is the speed of the fluid; and μ is the viscosity coefficient of the fluid. Reynolds number represents the ratio of the inertial force to the viscous force of a fluid. When the Reynolds number of a fluid is less than 2300, the fluid is in the state of a laminar flow. Another fluid-mixing-related parameter is Péclet constant, which is defined to bePe=Ul/D wherein D is the diffusion coefficient of molecules, and U is the speed of the fluid, and l is the length. Péclet constant represents the ratio of the convection to the diffusion of a fluid. In a macroscopic flow field, a turbulent flow is usually used to implement mixing; however, it no more works in a microscopic laminar-flow system. For a laminar flow, the mixing among different fluids results from diffusion. Nevertheless, the effect of molecular diffusion is much smaller than that of turbulence. Laminar mixing, also referred to as molecular diffusion, occurring inside a channel of only 200 μm wide, no uniform mixing can be obtained even after centimeters for mixing. Such a problem is one of the challenges micromixers have to confront.
Simply speaking, mixing can be regarded as the result of molecular diffusion and can be described with Fick's law for diffusion, which is defined to be:J=−AD∇c wherein J is diffusion flux; A is the contact area between two mixed fluids; D is the diffusion coefficient of the molecule of the fluids; c is the concentrations in the fluids; ∇c is the concentration gradient between the fluids. Adjusting the contact area between two mixed fluids or the concentration gradient between the fluids is able to improve the mixing effect; however, the concentration gradient is hard to control. Therefore, the main stream of the current micromixers is focused on enlarging the contact area between two mixed fluids.
The fluid in a microchannel has a pretty high ratio of surface area to volume. Via the structures of geometry, wall grooves, and barriers of a microchannel, secondary flows will be created to influence on the fluid. The flowing mode mentioned can generate massive foldings and stretchings of the fluid and make progress for mixing. Refer to FIG. 1 for a conventional micromixer (WO Pat. Ser. No. 03/011443 A2). In such a well-known passive micromixer 10, grooves 12a, 12b, 12,c, 12d, 12e, and 12f of a special geometrical structure are formed on the bottom wall of the mixing chamber 11 via a lithographic process. This special geometrical structure can create velocity vectors vertical to the flow direction of the fluid to form the helical flow for better mixing by way of the effects of foldings and stretchings.
Refer to FIG. 2 for a perspective view of a special embodiment of the conventional micromixer shown in FIG. 1—a staggered herringbone micromixer 20—and the helical flow field thereof. In the staggered herringbone micromixer 20, the bottom wall of the mixing chamber 23 has periodic and asymmetric structures 21a and 21b, which can generate two sets of vortices rotating in opposite directions. In the first semi-period, the right vortical bulb 22a is smaller than the left vortical bulb 22b as the asymmetric structure 21a is deviated and rightward (The positive x-axis is the right side, and the negative x-axis is the left side.). In the second semi-period, the right vortical bulb 22c is greater than the left vortical bulb 22d as the asymmetric structure 21b is deviated and leftward. After several cycles, the reciprocating vortical motions enable the fluid to be mixed uniformly. The staggered herringbone micromixer is satisfactory, however, it needs a 3 cm-channel-length to achieve the 90%-mixing-efficiency when the mixing channel is 200 μm wide and 70 μm high. Therefore, the present invention proposes a new micromixer to shorten the length down to millimeter-scale.